Circuit for capacity microphones or pickups



Ey M. G. NIQHQLSQN, .JR -F CIRCUIT FOR CAPACITY MICROPHONES 0R PICKUPS Filed D60. l5, 1945 ATTORNEY Patented July 1 8, 1950 FOR CAPACITY MICROPHONES R PICKUPS Madison G. Niemann, Jr.,*snyder,.1v. Y., assigner to' Colonial Radio Corporation, Buffalo, N. Y.'

application meenten 15, 1945, serai No, 635,215

vz claims. (cl. ssa-56) 1 This invention relates to improved operating circuits for condenser. microphones, condenser phonograph pickups, and similar apparatus', emplaying a pair of capacity elements-which are vibrated mechanically or acoustically to produce an electrical wave which is more or less a counterpart of the acoustic or mechanical vibrations.

Apparatus of this type as conventionally constructed and operated suffers from certain defects, among which may be mentioned an undesirably high ratio of noise to desired signal, the necessity for extremely careful and, therefore,

4expensive filtering, and. in cases where the microphone or pickup is used to produce modulation of radio frequency currents, a relatively low percentage of modulation.

Fig. l is a circuit diagram of a circuit in accordance with my invention, employing a condenser microphone or pickup to feed an audio amplifier, v y

Fig. 2 is a similar diagram showing one circuit whichl may be employed for producing amplitude modulation of a, radio frequency oscillation by the condenser microphone or pickup,

Fig. 3 is a circuit similar to Fig. 2, but modified in a manner-to provide an increasing percentage o f modulation over that of Fig. 2, and

Pig. 4 is a cross-section of a, condenser microphone or phonograph pickup in accordance with the principles of my invention.

In accordance with my invention, I eliminate or greatly reduce all of these'drawbacks. By the application of my invention, I am able to produce y an operating circuit employing a condenser microphone, phonograph pickup, or the like, which will deliver an electrical output, the signal component of which is extremely high compared to the noise component (high signal-to-noise ratio), in which little or no filtering is required,'and' with which, when thevariahle capacity element is employed as a modulator of radio frequency oscillations, 100% modulation may veasily be obtained, and all of this at little or no increase in Y cost over existing arrangements. and in some cases a considerable decrease in cost.

It will, therefore, be seen that among the objects of my invention are:

To provide a capacity-condenser microphone or pickup circuit which will deliveran electrical output having a high signaI-to-noise ratio.

To provide such apparatus which is relatively free from the conditions which cause noise in conventional circuits.

To provide such apparatus in which, although the actual capacity of the phonograph pickup orA microphone may be very small in comparison with the remaining'capacity of the circuit, high percentages of modulation up to 100% may be obtained.

Still-other objects and advantages of my invention will be apparent from the specication.

The features of novelty whichI believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its fundaental principles and as to its particular embodiments, will best be understood by referenceto the specification and accompanying drawing, in which In'circuits of the type to which this invention is addressed, it has heretofore been customary to use a shielded and grounded cable to protect the output from the condenser microphone or pickup, up to the point where it is impressed on the input circuit of the amplifier, which is usually one or more stages. each stage employing a thermionic vacuum tube, usually of the highV y gain time.

` Ordinarily the polarizing voltage is applied to `the shielded lead which extends from the microphone or pickup to the control gridof the amplifier'. Any leakage occurring from this conductor to the shielded cable manifests itself as noise and is likely to be serious, since it occurs in the circuit to lwhich the control grid of the amplifier is connected.

Moreover, because of this connection, it has heretofore been necessary izo-provide very effective filtering on the polarizing voltage supply side, because any hum or other irregularity in polarizing voltage is impressed directly upon the grid of the tube and amplified.

A further defect of conventional circuits is that the capacity and the variation in capacity obtainable in the condenser microphone and especially in the condenserrphonograph pickup is likely to be only a very small part ofthe total capacity of the circuit and, therefore, only a very low percentage modulation is obtainable, this resulting in low -eiciencies of transmission and reception of the modulated waves.

Referring now more particularly to Fig. l, in which li have illustrated the application of my invention to a circuit in which the microphone or pickup feeds an audio frequency amplifier,

l0 represents the condenser microphone, phonograph pickup, or the like. Polarizing voltage may be impressed vupon one electrode of condenser i@ through conductor ll, which may be connected to a suitable source, such as +B. The

atrasos may be connected to ground through resistor Il' shunted by condenser i8, and anode Ita may be connected through resistance I9 to +B, -B being connected to ground.

The audio frequency output o1' ampliier i5 may up I0 be operated to produce amplitude modula y be taken oii through condenser 2li to any suitable circuit. such as the second stage of ampliilcation, or may be impressed upon an indicator, such as a loud speaker (not shown). Surrounding conductor Ii there is provided grounded shield I3. and surrounding vconductor I2 a similar grounded shield IB, these shields extending as nearly as possible to the ends of their conductors. y

While, for simplicity of illustration, the shields are indicated as cylindrical elements, it will be understood that ordinary shielded cable may be employed, conductor II andits shield I3 and conductor I2 and-its shield It being simply pieces oi v ordinary shielded wire or cable, such as is available commercially. l

In operation, sounds falling on condenser It when it is a microphone. or vibration of one oi the elements when itis a phonograph pickup, vary the capacity of condenser I and thereby vary the voltage between the control electrode and cathode of tube l5. This variation in voltage is amplified by tube I5 and delivered to the output through condenser 20, or it may be further amplified or impressed upon a loud speaker.

It will be observed, 'with reference to Fig. l., that the high potential has been removed from conductor I2. which is connected directly to control electrode IEg of tube I5 and, therefore, there is no possibility of high voltage leakage from conductor I2 to shield Il. which would produce noise. Since the only voltage appearing on conductor I2 is relatively very small, there is little likelihood oi.' any noise being produced by leakage from this conductor. Leakage may, of course, occur between conductor II and shield I 3, but it will be observed that this will not impress any noise voltage on conductor I2 and the control electrode of tube I5.

Because the polarizing voltage no longer appears on conductor I2, it is no longer necessary to provide the expensive filtering heretofore necessary. Any B supplyf'which will give satisfactory operation or tube I5 will in general give satisfactory operation as a source of polarizing potential when applied to conductor I I. Y

To preserve the beneilts thus far obtained, it is desirable that no leakage occur between the elements or electrodes of condenser III, and this is best accomplished by mounting each plate or element on a separate insulator. these insulators in turn being mounted on a metal plate maintained at tlie circuit ground potential. Such an arrangement is shown in Fig. 4, in which electrodes 30 and II represent the condenser or pick- UP capacity plates.

Plate 3l may be mounted in insulating ring l2, and platell in insulating ring ll, and each o! these insulators is separately mounted in conducting plateu, connected to ground. Thus, any

' leakage which occurs in the microphone or pickup will be from respective elements tl-II to ground,

and there Iwill be no leakage between elements Ilandl.

t' Referring now more particularly to Fig. 2, In

which like reference numerals designate like parts.

as in Fig. 1, it is desired that microphone or picktion of a radio frequency oscillation. The radio frequency oscillation to be modulated may be obtained from oscillator 2l, which may be of any suitable type, supplying oscillations to inductance 22, coupled to inductance 23 .connected between one end of conductor. II and ground, the other end of conductor II being connected, as before. to one side of condenser I0.

Tube I5 may now be replaced by tube 24, which may be a detector. herein illustrated as a pentode, having cathode 24e. heater 24h, control grid 24g. screen grid 243g, suppressor 24sr, and anode 24a. For simplicity of illustration, the connections of screen 2dsg and suppressor 2481' are omitted since they are well known.

With this circuit it will be observed that the capacity oi neither shielded cable I3 nor Il adds to the transmission of voltage to detector 24. the residual transmission being dependent only upon the residual capacitance of the plates of condenser I0. The percentage modulation obtainable from this circuit is equal to the percentage variation in capacity oi condenser III caused by sounds falling upon it, or by the action oi the needle riding in the record groove.

A typical value of residual capacitance in such a circuit is 1 mm1.. while the variation of the capacity may be as lowvas 0.1 mm1'. Thus, the percentage modulation oi' the wave arriving at detector 24 may be as much as 10%, whereas with conventional circuits, such as heretofore' employed, the percentage would be much less than 1% i'or a typical shielded cable capacity of 50 mmf. h v

However, stili higher percentages oi modula tion are obtainable if the circuit shown in Fig. 2 is modiied to the form shown in Fig. 3. In this instance. the residual value oi.' the capacity of microphone or pickup Il may be nearly balanced'out as by grounding inductance 2l at or near its midpoint, connecting one end of inductance 23 to conductor II as before, and connecting the other end of inductance 23 through condenser 25, nearly equal to the value 'of the residual capacity or condenser Il, to conductor I2 at or near the point or its connection to the control electrode of tube M.

The balance is not at all critical because only a part oi the residual capacity oi condenser I0 is being balanced out, and the capacity of shields I3 and Il does not enter into the balance.l

In the specification I have explained the principles oi' my invention and the best mode in which I have contemplated applying those principles, so as to my invention from other inventions; and I have particularly pointed out and distinctly claimed the part. improvement. or combination iwhich I claim as mv invention or discovery.

While I have shown and described certain preferred embodiments of my invention, it will be understood 'that modications and changes may be made .without departing from the'spiritbination, a source of radio frequency oscillations to be modulated. an electro-'mechanical translating device having a pair of capacity elements, a thermionic vacuum tube having a cathodefa control electrode, and an anode, a shielded conductor extending from one of said elements to said control electrode, an inductance having the intermediate point thereof grounded, a second shielded conductor extending from one end of said inductance to the other of said capacity elements, and a conducting pathextending from the other end of said inductance to said control electrode, said path having interposed therein a capacity of such magnitude as to partially balance out the mean capacity oi said translating device.

2. The combination claimed in claim 1, in which said last mentioned capacity is adjustable to control the percentage modulation.

MADISON G. NICHOLSON, Jn.

REFERENCES crrnn The following references are of record in the tile of this patent:

UNITED STATES PATENTS Number Name Date 1,467,596 Wold Sept. 11, 1923 1,639,000 Horton Aug. 16, 1927 1,817,612 Craig Aug. 4, 1931 1,872,858 Whiting Aug. 23, 1932 2,368,036 OBrien Jan. 23, 1945 2,371,373 Badmaieff Mar. 13, 1945 2,386,049 Hausy Oct. 2, 1945 2,412,023

Woll Dec. 3, 1940 

